The greenhouse industry is experiencing an era of rapidly advancing technologies for artificial illumination. LED based luminaires have entered commercial use as grow lights relatively recently. HPS and conventional arc light sources are now moving aside and more efficient LED luminaires are emerging into markets including advanced functionalities e.g. integrated pest management (Vänninen et al., 2012).
However, the potential modes of LEDs for illuminating plants are still rarely fully optimized. Currently used LED based luminaires still suffer low efficiency and provide emission spectra not well overlapping with the absorption spectra of photo biological processes such as photosynthesis. Over-exposing of plants with high intensity sources and lack of advanced control modes such as pulsed illumination are still topics not fully researched or solved in practice. A LED spectrum can be matched with photo biological requirements to enhance plants' growth and to increase the total organic output i.e. the harvested volume of a greenhouse products e.g. tomato or lettuce. Photo biological requirements are mainly defined by the absorption spectrum of the photosynthesis and other photo biological processes in question. There is also a need to meet the timing requirements of the illumination when operating with a pulsed light. The timing requirement arises from the chlorophyll B excitation and electron transfer delay to the chlorophyll A associated process and the potential to optimize the energy usage for driving the photosynthesis. Other natural parameters that account for the illumination requirements include e.g. partial pressure of carbon dioxide, irrigation level of soil, temperature and type of canopy. Other requirements that constitute to the required illumination spectrum may arise e.g. from marketing motives to grow vegetables with certain skin colors or the need to enhance the product's nutrition content or other effective substance.
Different plants and biomass applications require slightly differing type of illumination conditions to reach optimal growth. This induces greenhouse industry to invest on many types of artificial grow lights. It is the objective of the disclosed invention to provide an integrated LED structure with adjustable emission characteristics to meet the different requirements of various biomass growing applications. The integrated structure, with densely packed emission areas, produces high spectral uniformity in the far field.
A good example is e.g. the growth of red and black soybeans. CN103947470A and CN103947469A disclose light spectrum conditions preferred for optimum growth of red and black soya beans, with rough blue, red, and yellow spectrum band ratios being 3:1:5 and 4:3:3 respectively, demonstrating the need for adjustable spectrum type light source to enable one artificial grow light to be used with a variety of different plants. Similarly for example tomato plant and spruce require quite different type of light to grow efficiently. The required spectrum components also vary between different growing cycles of a same plant e.g. during vegetation phase blue rich light is preferred, and flowering and fruit grow phases are typically connected with red rich light. Another requirement for adjusting the spectrum of the grow light is the need to grow e.g. vegetables with varying skin colors of e.g. bell paprika for marketing purposes or for enhancing certain nutrition components in the paprika fruit.
A grow light with adjustable spectrum would also allow new functionalities not yet fully exploited in the greenhouse industry. For example it is known that a pre-harvesting treatment of kale affects strongly on the nutrition content (Carvalho et al., 2014; Lefsrud et al., 2008). Another example is the UV flash-treatment of cultivated mushrooms prior harvesting or post harvesting to enrich their vitamin D content (Beelman et al., 2009).
Another example of potential benefits of a source with an adjustable emission spectrum becomes apparent from Nicklish, 1998, when considering biomass growth applications such as algae. The absorption spectrum shifts from around 680 nm peak towards lower wavelength peak around 630 nm when the photoperiod becomes shorter. Similar shift in absorbance is documented in the art (Eytan, 1974). The ratio of chlorophyll A and chlorophyll B concentration has been shown to change in time when plant is subjected to continuous illumination as e.g. in case of Red Kidney bean plants (Argyroudi-Akoyunoglou, 1970). Such change presupposes an alteration in the emission spectrum to maintain optimum growth conditions.
It is clear that the grow light should allow flexible modification of spectrum characteristics to enable its use for growing different types of plants and even modifying spectrum characteristics during the different growth phases. These requirements combined with the idea of growing biomass with a pulsed light source are now tackled with the disclosed invention.
Two main approaches exist to build a LED source for luminaires used as grow lights.
In the first approach, the emission spectrum can be generated by combining optical output of different color discreet LEDs. This type of hybridized LED structure is often called an RGB LED. In this approach the LEDs are discreet LED components and e.g. blue-red emissions have clearly distinct spatial source points. The light is produced within the compound semiconductor pn-junction while the emission spectrum from a single pn-junction is relatively narrow, typically only 10 to 40 nm. Due to narrow emission spectrum several semiconductor chips are used in combination to provide the required wider spectrum to fully cover the red and blue wavelength bands of the visible spectrum required by e.g. photosynthesis. Required semiconductor chips can be packaged discreetly or mounted inside a same package however optically forming still a large source point.
In the second approach the emission spectrum is generated within a single LED package. In this case one or several LED semiconductor chips excite wavelength conversion material or typically a phosphor material layer to generate continuous emission spectrum matching closely with the photo biological requirements. For example 440 nm LEDs chip excite an appropriately selected phosphor material layer and can provide typical double peak spectrum offering a relatively good match with the above explained requirements with the primary photo biological process of photosynthesis.
In short, commercial light sources, being LED, fluorescent or HPS, all still commonly apply continuous light with fixed optical spectrum. It is known that it would be beneficial to apply pulsed light to firstly save energy and secondly to apply light source that would enable spectrum adjustment to meet changing spectral requirements during the plant growth cycles, or phase of photosynthesis, or to allow use of same luminaire supporting varying light requirements. Pulsed light arrangement has been shown to benefit also algae growth (Sforza et al., 2012).
PPF (photosynthetic photon flux) should be kept at level similar or equal to sun light level that is roughly 2000 μmols/m2/s to avoid excess light and stressing plants. Now this applies for continuous light. With pulsed light the situation changes as the dark cycle can be adjusted so that the photo biological process has time to ‘use’ the light energy absorbed during the light cycle. Thus the maximum light intensity can be increased substantially from nominal sun light level of 2000 μmols/m2/s e.g. to 10000 μmols/m2/s to allow even faster growth. However, such arrangements presumes considering the excess heat from the light source, other growth limiting parameters such as the level of carbon dioxide, and also how to avoid self-shadowing from the canopy to best utilize high intensity source.
Artificial grow lights have been under research (Olle et al., 2013; Klueter et al., 1980; Yeh et al., 2009) and development for decades and also pulsed light sources have been introduced earlier, such as JPS6420034A. This source was based on discharge lamps and was able to produce pulse lengths between 1 to 50 ms. Drawbacks of this early innovation was that discharge lamps did not meet well the required spectrum characteristics as large part of the light energy is emitted at wavelengths not needed by photo biological processes. And the pulse lengths were not short enough to fully exploit the benefits of pulsed light.
A study reported by Tennessen with co-workers (Tennessen et al., 1995) shows the benefits of pulsed light. In this study the pulse period of 100 μs and dark periods of few ms were used. The experimental light source was assembled from discreet LED components emitting at narrow fixed wavelength bands of 658/668 nm only.
First pulsed grow light based on LEDs appears in U.S. Pat. No. 5,012,609. This approach was based on discreet emitters for each required wavelength band i.e. 400-500, 620-680, and 700-760 nm. The driving circuit was able to produce pulses in duration of 100 μs, i.e. at optimum length. However, the driving circuit was based on current-limiting-resistor and is considered to have a modest energy efficiency when compared to modern solutions such as the one disclosed in the disclosed invention. Main drawback of the approach was that it did not provide means to adjust the spectrum for different growth cycles. The spectrum was fixed as the discreet visible range wavelength emitters were all required to be in the same serial-parallel circuit.
U.S. Pat. No. 5,278,432 presents some innovations on the packaging and mounting of discreet LEDs on heat sinking substrate. However, driver circuit is still in the form of current-limiting-resistor and the spectrum is fixed with all emitters coupled in series-parallel fashion, excluding the possibility to somehow control the intensity at certain wavelength bands or to adjust the emission spectrum.
WO02067660 (A1) discloses a system level arrangement of red and white light LEDs to optimize the emitted spectrum to speed-up the plant growth. In the disclosed structure the spectrum is fixed after the discreet LEDs have been mounted on the carrier substrate. It is clear from this and later publications discussed below that the pulsed light is preferred mode of operation to reduce the total growth time.
A LED arrangement with an AC driver (cf. U.S. Pat. No. 8,410,725) provides means to reduce total cost of the system by applying same driver circuit for two discreet light sources, emitting in opposite phases of the sinusoidal AC current. Obvious issue is the required large spatial separation of the two LED strings to avoid over exposing the plants under lights and to gain the benefits of the pulsed lighting.
U.S. Pat. No. 8,302,346 discloses a growth enhancing system with a feedback based arrangement applying pulsed light source based again on discreet LED chips each emitting a fixed spectrum.
CN201797809 discloses light source arrangement that applies discreet LED emitters to form the required total spectrum including UV, UVB, blue and near IR.
CN103947470A, CN103947469A disclose light spectrum conditions preferred for optimum growth of black and red soya beans, with rough blue, red, and yellow spectrum band ratios being 3:1:5 and 4:3:3 respectively, demonstrating the need for adjustable spectrum type light source to enable wider use for growth of different plants.
US20130139437 discloses hybridized light source arrangement that would allow spectrum tunability and pulsed operation mode to prevent photosynthesis saturation. However, the presented light source structure has a system level approach based on discreet LED components mounted on printed circuit board with different emission wavelengths, and with a fixed ratio of LED emitters at individual wavelength ranges to create required spectrum. The expensive feedback system approach based on absorption and/or fluorescence sensing gives coarse feedback to allow tuning of intensity, and of the light on and off periods i.e. the light patterns. However, as the absorption of other than chlorophyll molecules such as carotenin molecules, play important role in ‘plant's’ heat sinking capability, and effectively large part of light energy is wasted when absorbance is used as a feedback. In our disclosed invention a system approach has been adopted that is based on preset or programmable pulse patterns.
WO2014188303 discloses means for enhancing plant growth by adjusting the ratio of blue and red lights alone. US2014152194A1 discloses another system to be able to provide necessary spectrum bands for enhancing the growth.
U.S. Pat. No. 8,549,787 provides an LED arrangement with fixed intensity ratios of characteristic peaks at wavelength bands of 400-500 nm (blue), 500-600 nm (green), 600-800 nm (red), and with 500-600 nm band to have lower intensity compared to other two. However, the said arrangement does not allow adjusting the ratio between the intensities of the said blue and red wavelength bands.
US2014034991A1 and U.S. Pat. No. 7,350,933 both disclose similar LED arrangement to each other that enable the tuning of the color coordinates and thus the chromaticity of the light emitted from the LED arrangement. However, these arrangements are not addressing the requirements of biomass growing applications or e.g. pulsed light operation. The emission spectrum is not meeting the photo biological requirements. The operation is defined to be continuous, while not meeting the requirement of having alternating emission spectrum of pulsed type.
WO2013141824A1 discloses a similar LED arrangement that enables the tuning of the spectrum for matching the chlorophyll b and a absorbance. However, the arrangement is not addressing other requirements of biomass growing applications such as the pulsed light operation. The operation is defined to be continuous failing to benefit from alternating emission spectrum.